Spin-Label Studies of Heterogeneous Polymer Systems

Department of Chemistry, University of Aberdeen, Old Aberdeen AB9 2UE, Scotland, UK 

The spin-label and spin-probe techniques have been used to study a wide variety of polymers, both in bulk and in solution, and several reviews of the subject have been published [1-3]. The object of this article is to provide a brief outline of the theoretical background, with particular reference to spin-labelling, bdbre discussing some recent applications of the technique to polymers in heterogeneous systems. 

The shape and width of the electron spin resonance (ESR) spectrum of a spin label or probe is sensitive to the mode and rate of rotation of the radical. Thus, examination of the ESR spectrum of the labelled or probed polymer can yield information on the dynamics and relaxations of the polymer and is therefore complementary to such techniques as mechanical, NMR and dielectric relaxation measurements. 

In the spin-probe experiment the free radical is simply dispersed in the polymer matrix. However, as any interaction with the polymer is only by secondary valence forces, the motion of the probe may not directly reflect the motion of the polymer. Spin-labelling, where the radical is covalently attached to the polymer chain, does give information on the dynamics of that part of the polymer to which it is joined. It is possible to label polymers specifically at either inner or terminal segments, and thus, provided that the label does not rotate independently or perturb the motion of the polymer, specific information about the dynamics of these particular sites can be obtained. An advantage of the ESR technique is its high sensitivity, so that, in favourable circumstances, spin concentrations as low as 10 M may be used. In practical terms, this relates to approximately one spin label per polymer chain. 

Polymer Spectroscopy. Edited by Allan H. Fawcett 1996 John Wiley A Sons Lid 

---

SPIN-LABEL STUDIES OF HETEROGENEOUS POLYMER SYSTEMS... 

A review of the application of ESR to the study of free radical polymerisation is given by Yamada and co-workers [146]. A survey of the application ESR spectroscopy spin label/probe methods in heterogeneous polymer systems is provided by Veksli and co-workers [147]. Spin probe methods allow the study of the MD of the polymer, its free volume, phase separation and phase morphology. 

Among the first heterogeneous systems involving polymers to be studied by the spin-label technique were solids suspended in polymer solutions [S, 9, 27-29]. The aim of these studies was to obtain new and complementary information on the nature and mechanism of polymer molecule adsorption at the solid-solution interface. In most of these studies the labelled polymer adsorbed from solution on the surface of adsorbents, such as silica, yield composite ESR spectra compris-... 

EPR is used extensively to detect, identify and follow the fate of radicals involved in polymerization or polymer degradation processes. It is also used extensively to characterize polymeric materials in terms of their morphology, heterogeneity, structural transformations, chain dynamics, and so on [2-5]. For this purpose, one can take advantage of the stable paramagnetic centers present in material to be examined (e.g., residual post-polymerization radicals, or TMIs used as catalytic centers or stabilizers). In most cases, however, external spin probes are added, including spin-labeled macromolecules (see Sections 23.2.1.5 and 23.2.2.1) [6]. The sensitivity and high content of structural information contained in the spin Hamiltonian parameters allow to obtain valuable - and often unique - data on the studied systems [7]. 

---